Back story

My tank bag was stolen recently along with the gloves inside. Besides the storage, the tang bag had a windowed pouch for a map or cell phone. I wanted:

a cheap solution that won't get stolen
secure and functional while riding
waterproof in case of rain
can be quickly and easily removed without scratching or leaving residue

Solution

A plastic, zippered pouch that attaches with magnets to the metal gas tank. In addition to a map you could store any paper you've got. If you plan to store a cell phone or wallet, test the stability with something heavier first.

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Step 1: Materials

A5 zippered pouches from a dollar store
neodymium magnets (minimum 4. I used 8 because mine are small.)
duct tape

Since I paid about 10 cents per magnet and 25 cents for the pouch ($2 for 8), total cost was about $1.

Step 2: Assemble and Test It

Tear a small piece of duct tape. Stick a magnet or two to the sticky side. Then stick the tape into the corner of the pouch. Repeat this for all four corners of the pouch.

Put your map or other papers inside. Attach it to your gas tank. You're done. Now go ride outside!

I tested this with a variety of papers and they were all very stable. This will solve my navigation problem for the rest of the summer until I can afford a new tank bag.